Electrical transformers typically have windings of conducting wire which must be separated by a dielectric (i.e. non-conducting) material. Usually the coils and dielectric material are immersed in a fluid dielectric heat transfer medium to insulate the conductor and to dissipate heat generated during operation. The heat-transfer medium must act as a dielectric as well. In a typical arrangement, cellulosic and/or aramid paper or board is used as the dielectric material. The cellulosic/aramid material is wrapped around the conducting wire, and used to separate the windings dielectrically, and may also be used as a structural support for the windings or other elements such as the cores. The fluid heat-transfer medium is typically an oil, which may be, for example mineral oil or a sufficiently robust vegetable oil.
During use of the transformer, the dielectric material and heat-transfer fluid are subjected to significant electromagnetic fields and significant variations of temperature and power surges and breakdowns. Over time, the relatively extreme conditions can lead to failure of the dielectric material and deterioration of the heat-transfer fluid. Deterioration leads to power loss due to dielectric loss, and may eventually lead to discharges and catastrophic failure of the transformer causing major pollution and/or fires. The dielectric and heat-transfer fluid can furthermore be directly and indirectly degraded by oxygen migration and water formation or ingression in the transformer.
Mineral oil generally shows excellent dielectric and heat-transfer behaviour, however, dielectric heat-transfer fluids are used in enormous quantities, (i.e. several hundreds of thousands of metric tons per year). The public becomes increasingly sensitive to environment and safety concerns around transformer units, and they are therefore subject to more and more stringent regulations. Many heat-transfer fluids currently used (such as mineral oil) pose a serious concern since they are flammable and do not biodegrade within reasonable time frame or simply not at all. Fluids coming from “bio” (i.e. living) sources are increasingly being seen as future fluids for those purposes. For example, U.S. Pat. Nos. 6,905,638 and 7,048,875 disclose transformers using vegetable oils as the heat-transfer fluid. The vegetable oil may contain chemically synthesised anti-oxidants.
A need remains for improved bio-degradable heat-transfer fluids, which are not limited to food grade oils, showing good performance over time.